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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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fmemopen(3) Library Functions Manual fmemopen(3)
fmemopen - open memory as stream
Standard C library (libc, -lc)
#include <stdio.h>
FILE *fmemopen(size_t size;
void buf[size], size_t size, const char *mode);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fmemopen():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
The fmemopen() function opens a stream that permits the access
specified by mode. The stream allows I/O to be performed on the
string or memory buffer pointed to by buf.
The mode argument specifies the semantics of I/O on the stream,
and is one of the following:
r The stream is opened for reading.
w The stream is opened for writing.
a Append; open the stream for writing, with the initial
buffer position set to the first null byte.
r+ Open the stream for reading and writing.
w+ Open the stream for reading and writing. The buffer
contents are truncated (i.e., '\0' is placed in the first
byte of the buffer).
a+ Append; open the stream for reading and writing, with the
initial buffer position set to the first null byte.
The stream maintains the notion of a current position, the
location where the next I/O operation will be performed. The
current position is implicitly updated by I/O operations. It can
be explicitly updated using fseek(3), and determined using
ftell(3). In all modes other than append, the initial position is
set to the start of the buffer. In append mode, if no null byte
is found within the buffer, then the initial position is size+1.
If buf is specified as NULL, then fmemopen() allocates a buffer of
size bytes. This is useful for an application that wants to write
data to a temporary buffer and then read it back again. The
initial position is set to the start of the buffer. The buffer is
automatically freed when the stream is closed. Note that the
caller has no way to obtain a pointer to the temporary buffer
allocated by this call (but see open_memstream(3)).
If buf is not NULL, then it should point to a buffer of at least
size bytes allocated by the caller.
When a stream that has been opened for writing is flushed
(fflush(3)) or closed (fclose(3)), a null byte is written at the
end of the buffer if there is space. The caller should ensure
that an extra byte is available in the buffer (and that size
counts that byte) to allow for this.
In a stream opened for reading, null bytes ('\0') in the buffer do
not cause read operations to return an end-of-file indication. A
read from the buffer will indicate end-of-file only when the
current buffer position advances size bytes past the start of the
buffer.
Write operations take place either at the current position (for
modes other than append), or at the current size of the stream
(for append modes).
Attempts to write more than size bytes to the buffer result in an
error. By default, such errors will be visible (by the absence of
data) only when the stdio buffer is flushed. Disabling buffering
with the following call may be useful to detect errors at the time
of an output operation:
setbuf(stream, NULL);
Upon successful completion, fmemopen() returns a FILE pointer.
Otherwise, NULL is returned and errno is set to indicate the
error.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ fmemopen(), │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
glibc 1.0.x. POSIX.1-2008.
POSIX.1-2008 specifies that 'b' in mode shall be ignored.
However, Technical Corrigendum 1 adjusts the standard to allow
implementation-specific treatment for this case, thus permitting
the glibc treatment of 'b'.
With glibc 2.22, binary mode (see below) was removed, many
longstanding bugs in the implementation of fmemopen() were fixed,
and a new versioned symbol was created for this interface.
Binary mode
From glibc 2.9 to glibc 2.21, the glibc implementation of
fmemopen() supported a "binary" mode, enabled by specifying the
letter 'b' as the second character in mode. In this mode, writes
don't implicitly add a terminating null byte, and fseek(3)
SEEK_END is relative to the end of the buffer (i.e., the value
specified by the size argument), rather than the current string
length.
An API bug afflicted the implementation of binary mode: to specify
binary mode, the 'b' must be the second character in mode. Thus,
for example, "wb+" has the desired effect, but "w+b" does not.
This is inconsistent with the treatment of mode by fopen(3).
Binary mode was removed in glibc 2.22; a 'b' specified in mode has
no effect.
There is no file descriptor associated with the file stream
returned by this function (i.e., fileno(3) will return an error if
called on the returned stream).
Before glibc 2.22, if size is specified as zero, fmemopen() fails
with the error EINVAL. It would be more consistent if this case
successfully created a stream that then returned end-of-file on
the first attempt at reading; since glibc 2.22, the glibc
implementation provides that behavior.
Before glibc 2.22, specifying append mode ("a" or "a+") for
fmemopen() sets the initial buffer position to the first null
byte, but (if the current position is reset to a location other
than the end of the stream) does not force subsequent writes to
append at the end of the stream. This bug is fixed in glibc 2.22.
Before glibc 2.22, if the mode argument to fmemopen() specifies
append ("a" or "a+"), and the size argument does not cover a null
byte in buf, then, according to POSIX.1-2008, the initial buffer
position should be set to the next byte after the end of the
buffer. However, in this case the glibc fmemopen() sets the
buffer position to -1. This bug is fixed in glibc 2.22.
Before glibc 2.22, when a call to fseek(3) with a whence value of
SEEK_END was performed on a stream created by fmemopen(), the
offset was subtracted from the end-of-stream position, instead of
being added. This bug is fixed in glibc 2.22.
The glibc 2.9 addition of "binary" mode for fmemopen() silently
changed the ABI: previously, fmemopen() ignored 'b' in mode.
The program below uses fmemopen() to open an input buffer, and
open_memstream(3) to open a dynamically sized output buffer. The
program scans its input string (taken from the program's first
command-line argument) reading integers, and writes the squares of
these integers to the output buffer. An example of the output
produced by this program is the following:
$ ./a.out '1 23 43'
size=11; ptr=1 529 1849
Program source
#define _GNU_SOURCE
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(int argc, char *argv[])
{
FILE *out, *in;
int v, s;
size_t size;
char *ptr;
if (argc != 2) {
fprintf(stderr, "Usage: %s '<num>...'\n", argv[0]);
exit(EXIT_FAILURE);
}
in = fmemopen(argv[1], strlen(argv[1]), "r");
if (in == NULL)
err(EXIT_FAILURE, "fmemopen");
out = open_memstream(&ptr, &size);
if (out == NULL)
err(EXIT_FAILURE, "open_memstream");
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1)
err(EXIT_FAILURE, "fprintf");
}
fclose(in);
fclose(out);
printf("size=%zu; ptr=%s\n", size, ptr);
free(ptr);
exit(EXIT_SUCCESS);
}
fopen(3), fopencookie(3), open_memstream(3)
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Linux man-pages 6.15 2025-06-28 fmemopen(3)
Pages that refer to this page: fopen(3), fopencookie(3), open_memstream(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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